Oilpan: fix build after r183426.

Source/modules/geofencing/GeofencingEvent.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/geofencing/GeofencingEvent.h"
 
 namespace blink {
 
 GeofencingEvent::GeofencingEvent()
 {
 }
 
 GeofencingEvent::GeofencingEvent(const AtomicString& type, const String& id, GeofencingRegion* region)
     : Event(type, /*canBubble=*/false, /*cancelable=*/false)
     , m_id(id)
     , m_region(region)
 {
 }
 
 GeofencingEvent::~GeofencingEvent()
 {
 }
 
+void GeofencingEvent::trace(Visitor* visitor)
+{
+    visitor->trace(m_region);
+    Event::trace(visitor);
+}
+
 const AtomicString& GeofencingEvent::interfaceName() const
 {
     return EventNames::GeofencingEvent;
 }
 
 } // namespace blink